Why does my car battery have wavering voltage values?

The other day I was working on my 99' Pontiac Firebird's headlight motors. I disconnected the battery and fixed the motors and everything seemed to be working fine. Now a couple of days later I notice the RPMs and the battery voltage meter on the dash both wavering. The car also completely shut off on me while I was stopped at a red light. I think I may have disconnected the positive terminal first on my battery while working on the car and this is causing my issues. What would have broken to cause my issues and is it easily replaceable or fixable? Thanks ahead of time :)